Power source for wire cut electrical discharge machining

ABSTRACT

In a power source for wire cut electrical discharge machining by which a pulsating voltage which is higher than a discharge voltage and has a quiescent time is applied to cause an intermittent discharge between the poles defined by an electrode wire and the material to be machined, the quiescent time or the discharge current continuing time is controlled in accordance with the number of times of a repeated discharge in order to restrict an average current flowing between the poles per unit time to a predetermined level to prevent the breakage of the wire and increase the machining speed.

TECHNICAL FIELD

This invention relates to a power source for wire cut electricaldischarge machining. More particularly, it provides a power source bywhich pulses having a controlled voltage which is higher than adischarge voltage are applied between the material to be machined and anelectrode wire extending therethrough (i.e., between the poles) to causean intermittent discharge to take place between the poles to effect wirecut electrical discharge machining.

BACKGROUND ART

A conventional power source for wire cut electrical discharge machiningis diagrammatically shown in FIG. 1. An electrode wire 10 is unwoundfrom a reel not shown, fed through an initial hole 100 in the material12 to be machined, and wound on a reel not shown. A condenser 14 isconnected between the electrode wire 10 and the material 12 in parallelthereto. A currentlimiting resistance 18 and a switching transistor 20are connected in series to a circuit connecting the condenser 14 and aDC power source 16. An oscillator 22 generates an ON-OFF signal whichturns on and off the switching transistor 20 to apply a pulsatingvoltage (and current) between the poles. According to the power sourcedevice shown in FIG. 1, the condenser 14 is charged through theresistance 18, and if the insulation between the poles is broken tocause a discharge, the energy stored in the condenser 14 is dischargedbetween the poles, while the charge remains stored in the condenser 14if no discharge takes place. The maximum average current (Imax) suppliedbetween the poles depends on the electrical conditions, such as the dutyfactor D and resistance R of a pulse from the switching transistor 20,and is expressed by the following equation: ##EQU1## in which E standsfor the voltage of the power source 16.

In electrical discharge machining, no discharge takes place immediatelyupon application of a voltage between the poles, but there usuallyoccurs a time lag which is called no-load time. The average current Iis, therefore, low during actual machining. The average current I isgenerally proportional to the speed at which the material 12 to bemachined is fed, and increases with an increase in the speed if theelectrical conditions are not changed.

Thus, the maximum average current Imax is the maximum value of thecurrent which can be supplied for the circuit of a power source for wirecut electrical discharge machining, and the average current value basedon the assumption that there is not any no-load time. The averagecurrent I is the average current obtained when there is some no-loadtime, i.e., during actual machining operation, and varies with theprogress of the operation. The current I has hitherto been about 8 A atmaximum.

As the speed at which the material to be machined is fed is increased,the average current I increases, and if it exceeds about a half of Imax,the machining operation becomes very unstable. In order to increase thematerial feeding speed, therefore, it is not sufficient to increase I,but it is also necessary to increase Imax. There is, however, a limit tothe current which can be supplied to the electrode wire 10, and if acurrent I₀ exceeding the limit is supplied thereto, the wire 10 isbroken. The threshold current I₀ depends on, for example, the materialand diameter of the electrode wire 10. The stability of the machiningoperation is obtained if the average current I is lower than thethreshold current I₀. In other words, there is no wire breakage if I andImax are lower than I₀. If Imax is lower than I₀, there is no fear ofthe wire being broken, even if I may become very close to Imax on rareoccasions due to an external disorder, or changes in the operatingconditions, such as non-uniformity in the thickness of the material tobe machined, or during the instability of operation which may occurduring the machining of a corner. If I exceeds about a half of Imax,however, the operation lacks stability; therefore, it is usuallypossible to supply only a current which is lower than a half of thethreshold current I₀, resulting in a reduction of the material feedingspeed to about a half of the ideal speed. The ideal speed is the speedat which the value of I is equal to that of I₀.

According to the conventional power source disclosed in Japanese PatentPublication No. 13195/1969, no condenser is connected between the polesin parallel thereto, but an electric current is supplied between thepoles directly by the ON-OFF operation of a switching transistor.According to this system, an electric current is supplied between thepoles for a predetermined length of time if the appearance of adischarge is detected by application of a voltage between the poles, andthen, the supply of the current is discontinued for a predeterminedlength of time. This system makes it possible to control uniformly thepeak value of the discharge current supplied between the poles after theappearance of a discharge has been detected, and the duration of itssupply, and thereby increase the machining speed in accordance with thesurface roughness of the material to be machined. This system, however,has the disadvantages hereinabove pointed out of the conventional powersource shown in FIG. 1. If I exceeds about a half of Imax, the operationbecomes unstable; therefore, it is possible to employ only a currentwhich is lower than a half of the threshold current I₀ and achieve amaterial feeding speed which is only lower than about a half of theideal speed.

DISCLOSURE OF THE INVENTION

In view of the problems hereinabove pointed out, this invention providesa power source for wire cut electrical discharge machining which makesit possible to prevent the breakage of an electrode wire and increasethe machining speed. The time for which the supply of a dischargecurrent is discontinued, or the time for which it is continued iscontrolled in accordance with the frequency of repetition of anelectrical discharge so that the average current supplied between theelectrode wire and the material to be machined per unit time may berestricted to a predetermined level lower than the threshold current atwhich the wire may be broken during the machining operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a conventional power source forwire cut electrical discharge machining;

FIG. 2 is a schematic diagram showing a power source for wire cutelectrical discharge machining embodying this invention;

FIG. 3 is a diagram showing by way of example an oscillating circuitwhich is used in the apparatus shown in FIG. 2;

FIG. 4 is a diagram showing by way of example a discharge detectingcircuit which is used in the apparatus shown in FIG. 2;

FIG. 5 is a diagram showing by way of example a discharge currentcontinuing time setting circuit which constitutes the oscillatingcircuit shown in FIG. 3;

FIG. 6 is a diagram showing by way of example a drive circuit whichconstitutes the oscillating circuit shown in FIG. 3; and

FIG. 7 is a diagram showing another example of the oscillating circuitused in the apparatus shown in FIG. 2.

BEST MODE OF CARRYING OUT THE INVENTION

An embodiment of this invention is shown in FIG. 2. Like numerals areused to indicate like parts in FIGS. 1 and 2. Referring to FIG. 2, apower source 16 and a switching circuit 24 are connected in seriesbetween the poles defined by an electrode wire 10 and the material 12 tobe machined. The switching circuit 24 comprises a parallel combinationof a plurality of sets of serially connected transistors Tr₁ to Tr_(n)and resistances R₁ to R_(n). An oscillating circuit 26 controls thecircuit continuity of the transistors Tr₁ to Tr_(n). A dischargedetecting circuit 28 detects the appearance of a discharge between thepoles by a voltage between the poles or a current flowing therebetween,and transmits a signal to the oscillating circuit 26 to drive it.

FIG. 3 is a diagram showing an example of the oscillating circuit 26 inthe apparatus shown in FIG. 1.

The operation of the apparatus embodying this invention will bedescribed with reference to FIG. 3. Referring to FIG. 3, if a voltage isapplied between the poles to cause a discharge to appear therebetween,the discharge detecting circuit 28 detects it and transmits a pulsesignal. This pulse signal is fed to a discharge current continuing timesetting circuit 30. The time for which an electric current is suppliedis measured by the circuit 30, and after a predetermined length of time,a reset pulse is transmitted to a flip-flop 32. If the flip-flop 32 isreset and the output of its Q terminal reaches the L level, the outputof a drive circuit 46 also reaches the L level, and the transistors Tr₁to Tr_(n) are turned off. The circuit 30 may comprise a clock circuitand a counter, or may alternatively comprise a one-shot multivibrator.

FIG. 4 shows a specific example of the discharge detecting circuit 28.The voltage detected from between the poles is put in at a terminal 72and compared by a comparator 74 with a reference voltage put in at aterminal 70, and the output of the comparator 74 is received by aone-shot multivibrator 76, whereby the appearance of a discharge betweenthe poles is detected. FIG. 5 shows a specific example of the dischargecurrent continuing time setting circuit 30. If a flip-flop 80 is set bythe output signal of the discharge detecting circuit 28 and the outputof its Q terminal reaches the L level, a counter 82 starts counting inresponse to a clock 84. If the counter 82 finishes counting a value setby a setter 86 and corresponding to a discharge current continuing time,an exclusive OR gate 87 and a NAND gate 88 transmit an output to theflip-flop 32. FIG. 6 shows a specific example of the drive circuit 46.It comprises a NAND gate 92 which receives a signal from a start switch90 and an output from the Q terminal of the flip-flop 32, and a drivetransistor 94 which drives the transistors Tr₁ to Tr_(n) in response tothe output of the NAND gate 92.

If the output of the Q terminal of the flip-flop 32 reaches the L level,a counter 34 is released from its reset position, and starts to countthe pulses supplied by a clock pulse generator 36. The frequency of thepulses generated by the clock pulse generator 36 depends on the setquiescent time. The quiescent conditions are set on a counter 38 as willhereinafter be described. The output of the counter 38 which istransmitted through an inverter 40, and the output of the counter 34 areput into an exclusive OR gate 42, and the output of the gate 42 istransmitted to the S terminal of the flip-flop 32 through a NAND gate44. The NAND gate 44 generates a pulse if the outputs of the counters 34and 38 coincide with each other. In other words, if the counter 34counts the output of the clock pulse generator 36 to a value set on thecounter 38, the NAND gate 44 generates a pulse, whereby the quiescenttime is counted. If the output of the NAND gate 44 is put in theterminal S of the flip-flop 32, the output of its Q terminal reaches theH level, and the transistors Tr₁ to Tr_(n) are turned on in response tothe output of the drive circuit 46.

The quiescent conditions are set on the counter 38 as will hereunder bedescribed. If the output of the counter 38 coincides with the quiescentconditions set by a latch 48, a NAND gate 50 provides an L level outputthrough an exclusive OR gate 49, and if not, the NAND gate 50 providesan H level output. A NAND gate 52 provides an L level output only whenall of the outputs of the counter 38 are of the H level (i.e., in thecase of maximum quiescence). Thus, the latch 48, exclusive OR gate 49and NAND gate 50 constitute a setter for the maximum quiescent time.

The counter 38 is designed to receive an up or down signal. If thecounter 38 receives an up signal, the quiescent time is prolonged, andif it receives a down signal, the quiescent time is shortened. In otherwords, the counter 38 constitutes a time setter of which the set timevaries with the output of a counter 54.

The counter 54 is an up or down counter designed for receiving an upsignal which is a discharge detecting signal from the dischargedetecting circuit 28, or a down signal which is a clock pulse signalfrom a clock pulse generator 56. The frequency f_(L) of the clock pulsegenerator 56, which corresponds to a limit current I_(L), is slightlylower than the frequency f₀ corresponding to the threshold current I₀which causes the wire to break. The limit current I_(L), the frequencyf₁ and the discharge current continuing time T_(ON) have a relationshipwhich is expressed by the equation: I_(L) =E/R×T_(ON) ×f_(L). Thefrequency f_(L) is determined by the duration of voltage applicationbetween the poles, diameter and material of the electrode wire andpressure of a machining fluid, and defines the limit value of adischarge frequency.

The counter 54 compares the number of times of a discharge and the clockpulse corresponding to the limit current I_(L) every moment. If thenumber of times of a discharge increases, the carry (CA) terminal of thecounter 54 generates an output pulse to set a flip-flop 64, and if thenumber of times of a discharge decreases, the borrow (BR) terminal ofthe counter 54 generates an output signal to reset the flip-flop 64. Thecounter 54 has a number of bits which is required for obtaining anaverage. Its sensitivity is lowered if the number of bits is increased,and raised if it is decreased. Thus, the counter 54 is a comparisoncounter.

If the flip-flop 64 is set, its Q terminal provides an H level output.This H level signal and a clock pulse from a clock pulse generator 58are transmitted to the UP terminal of the counter 38 through an AND gate60 and a NAND gate 62 to move forward the counter 38 to prolong thequiescent time. If the number of times of a discharge is greater thanthe output frequency of the clock pulse generator 56, the counter 38 isgradually moved forward until the maximum quiescence is obtained. If thenumber of times of a discharge is smaller, the borrow terminal of thecounter 54 generates a pulse, and the Q terminal of the flip-flop 64provides an H level output. This H level signal and the clock pulse fromthe clock pulse generator 58 are transmitted to the down (DW) terminalof the counter 38 to move the counter 38 backward to shorten thequiescent time. If the number of times of a discharge is smaller thanthe output frequency of the clock pulse generator 56, the counter 38 isgradually moved backward until the quiescent time coincides with thequiescent conditions set on the latch 48. The clock pulse generator 58is a factor which determines the timing for a change in the quiescenttime. If it has a short cycle, the quiescent time changes quickly. Theelectrode wire 10 does not break immediately with an increase incurrent, but usually when it has continued for, say, 50 or 60 msec.Therefore, the clock pulse generator 58 may have a cycle of, say, 5 or 6msec.

If the number of times of a discharge exceeds a predetermined level(i.e., the average current I exceeds the limit current I_(L)), thequiescent time is gradually prolonged so that the average current I maybe lowered to a level lower than the limit current I_(L). If stabilityis obtained, the quiescent time is gradually changed until it reachesthe quiescent time set by the notch. Thus, the maximum average currentImax that depends on the electrical conditions can be about twice higherthan the threshold current I₀ which causes the wire to break; therefore,the machining speed can be increased if the machining current I israised to a level nearly equal to I₀ during stable operation. It hasexperimentally been found that the power source of this inventionenables a machining speed which is about 1.5 times higher than thatwhich has hitherto been possible.

Although the apparatus shown in FIGS. 2 to 6 is designed for controllingthe quiescent time in accordance with the number of times of adischarge, it is alternatively possible to control the discharge currentcontinuing time by employing a circuit which is substantially equivalentto what has hereinabove been described.

FIG. 7 shows another example of the oscillating circuit 26 designed forcontrolling the discharge current continuing time. Like numerals areused to indicate like parts in FIGS. 3 and 7, so that no repeateddescription may be necessary. In the circuit of FIG. 7, the counter 38is a counter which sets the discharge current continuing time. Theflip-flop 32 is reset by the output of the NAND gate 44 so that its Qterminal may provide an L level output, and the drive circuit 46 turnsoff the transistors Tr₁ to Tr_(n). A flip-flop 102 is set by a signalfrom the discharge detecting circuit 28 so that its Q terminal mayprovide an L level output, and the counter 34 starts counting. The Qterminal of the flip-flop 32 transmits an output to a quiescent timesetter 100, and after the lapse of the set quiescent time, the setter100 transmits an output to the set terminal S of the flip-flop 32 sothat its Q terminal may provide an H level output to operate the drivecircuit 46. The setter 100 may be equal in construction to the dischargecurrent continuing time setting circuit shown in FIG. 5.

In case the discharge current continuing time is changed, control ismade to decrease the pulse width, as opposed to the case in which thequiescent time is changed. The control of the average current per unittime may, therefore, be performed by the automatic control of thedischarge current continuing time between the maximum (by the latch 48)and the minimum of the set values in accordance with the number of timesof a discharge.

This invention makes it possible to prevent the breakage of an electrodewire and increase the machining speed, since the quiescent time or thedischarge current continuing time is controlled in accordance with thenumber of times of a discharge to control the average current per unittime at a predetermined level lower than the threshold current whichcauses the wire to break.

We claim:
 1. A power source for producing a discharge voltage for awire-electrode type discharge machining apparatus, comprising:means forproducing a signal indicating presence of a discharge between a wireelectrode and a workpiece being machined with said wire electrode; afirst clock source for producing as an output clock pulses at a firstfrequency, said first frequency being set in proportion to a limitcurrent, said limit current being slightly less than a threshold currentat which said wire will break; a first up-down counter receiving on oneof count-up and count-down inputs thereof said signal indicative of saiddischarge and on the other of said count-up and count-down inputs saidoutput of said first clock pulse source; a second clock source forproducing as an output clock pulses at a second frequency, saidfrequency being set in accordance with a maximum allowed rate of changeof duty cycle of said discharge voltage; a second up-down counter; meansfor applying said output of said second clock pulse source to one of acount-up and a count-down input of said second up-down counter selectedin response to a count output of said first up-down counter; a thirdclock pulse source for producing as an output clock pulses at a thirdfrequency, said third frequency being set in accordance with a dutycycle of said discharge voltage; a third counter receiving as a clockinput said output of said third clock pulse source; means for comparinga count output of said second counter with a count output of said thirdcounter; means for producing a pulse signal occurring a predeterminedperiod of time following said signal indicative of said discharge; andmeans for turning ON said discharge voltage in response to an output ofsaid comparing means and OFF in response to said pulse signal occurringsaid predetermined period of time following said signal indicative ofsaid discharge. wherein said first, second and third frequencies are setsuch that an average current flowing between said wire electrode andsaid workpiece is restricted to a predetermined level.
 2. The powersource of claim 1, further comprising means for inhibiting changes in acount output of said second counter so as to maintain said duty cycle ofsaid discharge voltage within predetermined limits.
 3. The power sourceof claim 2, wherein said inhibiting means comprises:latch means forstoring a value indicative of a maximum quiescent time of said dischargevoltage; second comparing means for comparing said count output of saidsecond counter with said value stored in said latch means; and means forgating said count output of said first counter in response to acomparison output of said second comparing means.
 4. The power source ofclaim 3, wherein said inhibiting means further comprises means fordetecting a predetermined count output of said second counter, an outputof said detecting means being applied to an input of said gating means.5. The power source of claim 1, wherein said third frequency is set inaccordance with a quiescent time of said discharge voltage.
 6. The powersource of claim 1, wherein said means for turning said discharge voltageON and OFF comprises a flip-flop and a driver circuit receiving as aninput an output from said flip-flop, said flip-flop having one of setand reset inputs receiving said pulse signal occurring saidpredetermined period of time following said signal indicative of saiddischarge and the other of said set and reset input receiving saidoutput of said first comparing means.